This invention pertains to the art of dual capacity compressors or pumps in which the dual capacity is obtained through changing the connecting rod stroke length from one length to another through reversal of the driving motor.
While I consider that my invention is applicable to devices used in other environments, it is particularly applicable for use in compressors in domestic heat pumps especially designed for use in a northern climate where a wide range of refrigerant flow rate is required. If the heat pump is sized for the greatest heating load which occurs at low ambient temperatures, then it has too much capacity at more moderate ambient temperatures and must cycle on and off, which leads to performance penalties.
One arrangement which has been devised to avoid the problem is found in Sisk U.S. Pat. No. 4,236,874 which teaches a reciprocating compressor which employs an eccentric cam rotatably mounted on the crank pin in which the rotation of the cam on the crank pin is angularly limited. On reversal of compressor rotation, the cam shifts from one angular extremity to an opposite angular extremity and by so doing changes the vector sum of cam eccentricity and crank pin throw so as to provide two different stroke lengths. A change in displacement and clearance ratio results in two different refrigerant flow rates depending upon the direction of rotation of the motor. The cam is driven from one end point to the other by the fraction of compressor torque delivered by the cam eccentricity.
Experimental use of the noted arrangement has shown that it operates substantially in the manner intended, but has also shown that the operation of the compressor in the reduced capacity mode has certain deficiencies. In particular, as the pressure ratio across the compressor increases (due to a decrease in the temperature of the medium from which the heat pump is drawing heat), the flow rate which the compressor delivers drops in accordance with the amount of stroke reduction designed into the eccentric cam. This condition is caused by the larger clearance volume resulting from a shortened stroke. The larger the clearance volume, the smaller the pressure ratio the piston can produce. Because the friction and other losses in the compressor are not decreasing at a comparable rate, the compression efficiency of the compressor decreases rapidly along with the flow rate.
The aim of my invention is to provide an arrangement in which this loss of efficiency during operation in the reduced capacity mode is lessened.